This invention relates generally to tuners, and more particularly, to extremely accurate detented type UHF television tuners.
Detented type UHF television tuners utilizing a detented selector shaft which drives a continuously variable main tuning shaft by means of a speed reducing gear mechanism are well known. Such tuners have reached a high degree of refinement, with the more sophisticated ones of the tuners having a separate detent position for each of the seventy UHF channels that can be received and a digital dial mechanism that provides a precise nonambiguous indication of the number of the channel being received. In such UHF television tuners, the various components of the detenting mechanism and the gear mechanism must be fabricated with a high degree of precision to assure that the number of the channel actually received by the tuner corresponds to the number indicated by the dial mechanism.
One seventy-position UHF tuner is described in U.S. Pat. No. 3,842,683 issued to Alarico A. Valdettaro, and incorporated herein by reference. The UHF tuner described in the above-referenced Valdettaro patent is a 70-position detented type UHF tuner that has been designed to provide a tuning accuracy of approximately .+-.3 megacycles. Because the channel spacing between adjacent television channel allocations is six megacycles, such a tuner is sufficiently accurate to be usable with an indicator dial providing a unique read out for each television channel, particularly when employed with an associated AFC control circuit.
Recently, in accordance with its policy of requiring the operation of UHF and VHF television tuners to be alike, the Federal Communications Commission issued a ruling requiring that all detented type UHF television tuners have a tuning accuracy of .+-.1 megacycle over a seventy channel UHF range. Accordingly, there is a great need at this time to provide a commercially feasible UHF television tuner that complies with the recent FCC rulings.
It is, therefore, an object of the present invention to provide a new and improved UHF tuner arrangement which avoids one or more of the above-discussed disadvantages of the prior art arrangements.
It is another object of the present invention to provide a detented UHF tuner arrangement that provides greater tuning accuracy than tuners heretofore developed.
It is a further object of the present invention to provide a UHF television tuner that has a tuning accuracy of .+-.1 megacycle over the entire UHF range.
It is yet another object of the present invention to provide a highly accurate UHF television tuner that can be manufactured at a cost comparable to that of presently manufactured UHF tuners.
Briefly, in accordance with a preferred embodiment of the invention, the range of rotation of the main tuning shaft is increased from a nominal 180.degree. to a nominal 270.degree.; for example, 276.degree.. This permits a 4.degree. spacing between each of the 70 UHF television channels (69 spaces between 70 channels). This provides the dual advantages of reducing the amount of error in capacitance occurring as a result of a given error in position, and further reduces the errors introduced by the speed reducing drive mechanism by reducing the reduction ratio and consequently the number of gears necessary to achieve that lower reduction ratio.
Other factors such as wear, temperature variations and aging also serve to reduce the accuracy of a tuner. Accordingly, the housing of the tuner is rigidized by spot welding its corners. The corners are also soldered to provide a better electrical connection between adjacent sides of the housing than can be achieved by spot welding alone. The housing is provided with a cover that is also soldered at the corners further to increase the rigidity of the unit, and the depth of the housing is increased to reduce the effects of the cover on the electrical characteristics of the tuner.
The accuracy with which the tuning capacitor rotor is positioned with respect to the stator also determines the accuracy of the tuner. Accordingly, an improved bearing surface is provided for the end of the rotor shaft to improve the accuracy of the positioning of the rotor shaft and to reduce wear on the rotor shaft that causes the rotor blades to shift with respect to the stator blades during the life of the tuner. An improved stator mounting arrangement is provided to eliminate the shift in the position of the stator blades that can occur with time with conventional mounting techniques. An improved method for mounting the rotor plates on the rotor shaft is provided to permit the rotor plates to be more accurately positioned with respect to the stator plates, and an improved contact is provided between the rotor shaft and the tuner housing to provide a more effective grounding of the rotor shaft to further improve the accuracy of the tuner.
In addition to the factors described above, mechanical phenomena occurring in the drive train can contribute to the inaccuracies of the tuner. One such phenomenon is known as inertial creep. Inertial creep occurs in detent type tuners that have a main tuning shaft that is operated by means of a detented selector shaft and a continuously variable fine tuning shaft. In such tuners, the fine tuning shaft is coupled to the detented selector shaft by means of a clutch or the like, and the acceleration and deceleration of the detented selector shaft caused by the channel selection operation can cause a sufficient slippage of the clutch that interconnects the selector shaft and the fine tuning shaft to detune the tuner enough to require the user to periodically readjust the fine tuning. Accordingly, in accordance with an important feature of the invention, an improved detenting mechanism is provided for reducing the acceleration and deceleration of the selector shaft caused by the detenting mechanism to thereby reduce the inertial loads on the clutch interconnecting the selector shaft and the fine tuning shaft to thereby substantially reduce the amount of inertial creep produced.